1. Field of the Invention
The present invention relates to a Raman amplifier, optical pump for use in a Raman amplifier, and a method for amplifying optical signals in various optical media. More particularly the invention relates to a Raman amplifier, pump and method that employs a pump for a Raman amplifier to identify an acceptable range of stimulated Brillouin scattering (SBS) generated in the amplifier fiber that causes no more than a predetermined amount of relative-intensity-noise (RIN) deemed acceptable for system performance requirements.
2. Discussion of the Background
Much of the technical background and motivation for using Raman amplifiers and optical systems is described in U.S. Pat. No. 6,292,288, the entire contents of which is incorporated herein by reference.
From a system-level perspective, it is desirable to provide a high bit rate system for low cost. However, to support high bit rate systems, a higher signal to noise (SNR) is required at the receiver to provide an adequate bit error rate (BER). With regard to the SNR, the signal level may be increased by increasing the input power from the signal source. However, the benefit of increasing the signal power beyond a predetermined amount can be wasted (i.e., a power penalty) due to the non-linear effect of the optical fiber itself. In other words, for a given BER, the power penalty is the extra optical power required to produce the same BER than if there was no noise or interference added by the non-linear effect. Thus increasing the amount of signal power in an optical transmission is only one aspect to the overall system analysis for obtaining the maximum performance.
To help resolve the limitation of the non-linear effect, fiber Raman amplifiers have become useful. A Raman amplifier is beneficial from a systems perspective because it reduces the cost for regenerative repeaters by increasing the span for which repeaters are required. Increasing the span between repeaters reduces the overall number of repeaters in a system, thus lowering cost and increasing the system's mean-time-between failure.
Conventionally, backward pumped Raman amplifiers have commonly been used, where the propagation direction of the pump light is opposite the propagation direction of the signal light. In this way, variations in the pump light, do not prejudicially over amplify, or under amplify specific portions of the signal, thereby providing a relatively non-prejudicial gain to the envelope of the signal.
Forward pumped Raman amplifiers are also known to have some merit, vis-à-vis backward pumped amplifiers, such as to suppress deterioration due to non-linear effects and ASE noise. However, the industry has generally failed to adopt forward pumped Raman amplifiers because of several problems. First, additional noise is present due to the transfer of pump relative intensity noise (RIN) to the signal light. Unlike with backward pumping, in a forward pumped Raman amplifier variations of the pump intensity are transferred to the signal light. In order to combat this problem, low RIN lasers were developed. However, low RIN pump lasers were later found to cause a new problem of causing stimulated Brillouin scattering (SBS) of pump light when used in a Raman amplifier system. Consequently, pump RIN degrades (i.e., the amount of noise is increased) in the fiber when the pump causes SBS. As the pump RIN becomes larger, the amplified signal has more noise contained therein due to pump-to-signal RIN transfer, thereby degrading system performance. Furthermore, the presence of SBS especially at large levels means that some of the pump power is lost wasted, which leads to lower amplifier efficiency.
According to these limitations with forward pump systems, conventional design philosophy dictates that no pump SBS can be tolerated for use in an optical communication system. The conventional method for using a forward pumped system is to ensure that no pump SBS exists by confirming that the pump reflection power generally corresponds with the Rayleigh scattered power of the pump light launched into the amplifier fiber. Moreover, the Rayleigh scattered pump power level is present regardless of the existence of SBS.
Since both SBS and Rayleigh scattering are components of pump reflected power, if the pump reflected power is generally set to correspond with the Rayleigh scattering level, then the amount of SBS is effectively nothing.
Low RIN pump laser diodes typically have a narrower longitudinal mode spectral line width, which in turn causes larger SBS in the fiber than conventional broader line width pump LDs. Once again, because the conventional design practice is to set to zero SBS, low RIN pump LDs are not able to be used at their full power capacity, thus making low RIN LDs less effective than if SBS was deemed not to be a problem at all.
In the past, Ohki et al., “Increase of relative intensity noise after fiber transmission in co-propagating Raman pump lasers,” OAA2002, Paper PD7, did show pump RIN, after passing through the fiber amplifier, increasing due to SBS, but nevertheless a quantitative relationship between RIN increase and pump SBS was not identified.
After recognizing the linkage between low RIN and SBS, the present inventors recognized that the system analysis for a conventional forward pumped Raman amplifier (or co-pumped Raman amplifiers) may permit some amount of SBS provided that a better relationship was understood between SBS, RIN and system performance. Moreover, the present inventors recognized that by not appreciating the source of the exact relationship between RIN and SBS, it would not be possible to select pump LDs for a forward pumped or co-pump Raman amplifier and still provide maximum system performance, or make full use of the LD's pump power capacity.
A conventional technique for measuring SBS is shown in FIG. 1. A pump 2 provides optical pumping power to a fiber 9 by way of a monitor/coupler 8. The coupler 8 may, for example, 1% coupling so as to provide some predetermined amount of pump power (e.g., 1%) to an optical power meter 6, which serves as an input monitor. Power reflectivity is then monitored by a reflection monitor, which may be an optical power meter and/or an optical spectrum analyzer 4. The reflective power passes through the monitor/coupler 8 and is provided to the optical power meter or optical spectrum analyzer 4. It is possible to separate SBS, which is present at 0.1 nm on the longer side of each Rayleigh backscattering peak, from the reflected light, if an optical spectrum analyzer 4 could be used. As previously discussed, according to conventional practice, no pump SBS is tolerated, and to ensure no pump SBS exists, the conventional monitoring technique confirms that the pump reflection power is set to generally the same level as the Rayleigh scattered power.
Consistent with the selection of detection of SBS, is the use of power penalty for selecting acceptable LDs for use in a system. Typically, a power penalty specification is set and an associated power reflection ratio (PRR), which is a ratio of total reflected power to total input power, is used as a criteria for selecting suitable LDs. Since the relation between power penalty and PRR has not yet been shown, only the LD modules that provide acceptably low PRR (e.g., as low as that caused by Rayleigh backscattering) has been selected. Since LD modules are expensive, the cost of each “acceptable” part increases as the manufacturing yield decreases. Likewise, more LDs are deemed unacceptable when they produce a higher PRR than the selection criteria.